Doped tin oxides are known among metal oxides for their relative transparency and high electrical conductivity. These properties are advantageously employed in a variety of electro-optical applications, e.g., providing transparent conductive coatings on particles or surfaces. One such application is the fabrication of transparent electrodes on electrochromic display devices which typically have an electrolyte material in contact with an electrochromic material so that an electrochromic effect is generated when an electric potential is applied across the interface of the two materials. When electrodes are provided on both sides of the materials, e.g., in sandwich-like structure, the electrode on at least one side of the display laminate needs to be relatively transparent to permit observation of the electrochromic effect. In such devices typical electrochromic materials include tungsten oxide, Prussian Blue, polyaniline and viologens. Transparent electrodes have been fabricated by vapor deposition of antimony-doped tin oxide (ATO) coating on a glass or plastic substrate.
Although doped tin oxides have been employed as transparent conductors in electrochromic devices, it appears that the possibility that doped tin oxide might be useful as a practical electrochromic material has not been discovered. For instance, Orel et al. reported in the Journal of the Electrochemical Society, Vol. 141, page L127 (1994) that a film of ATO exhibited a change in light reflectance between a reduced and oxidized state of less than 5%, which corresponds to a contrast ratio (as defined hereinbelow) of less than 1.05. Because such a change in color is not readily discernable to the typical human eye, it has not been recognized or discovered that doped tin oxides have useful electrochromic properties.
A variety of dopants are used to make conductive metal oxides, some of which, e.g., fluorine-doped tin oxides are not known to exhibit any useful electrochromic effect regardless of modification. Similarly, ATO, when provided in film form, also does not exhibit any useful electrochromic effect. When select doped tin oxides, e.g., ATO and niobium-doped tin oxide, are provided in particle form in an electrochromic generating environment, a surprising electrochromic effect is achieved. Thus, this invention is directed to the surprising discovery that certain of the conductive doped tin oxides can be useful high contrast electrochromic materials and to electrochromic devices employing such electrochromic doped tin oxides.